Shape memory alloy actuators

ABSTRACT

Linear actuators comprised of a plurality of geometric links connected together in displacement multiplied fashion by a plurality of SMA wires. The links may have a trigon or chevron configuration. The trigon links may be combined with a hexagonal or rhomboidal shaft to create a defined stacking pattern of links about the shaft. The shaft extends from the medial portion of the stack. Ohmic heating circuits connect to non-moving ends of SMA wires. Various groupings of links in parallel displacement are described.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a continuation-in-part of application Ser.No. 10/056,233, filed Dec. 3, 2001, and issued as U.S. Pat. No.6,762,515 on Jul. 13, 2004, which is a continuation of application Ser.No. 09/566,446, filed May 8, 2000 and issued as U.S. Pat. No. 6,326,707on Dec. 4, 2001, for which priority is claimed.

FEDERALLY SPONSORED RESEARCH

[0002] Not applicable.

SEQUENCE LISTING, ETC ON CD

[0003] Not applicable.

BACKGROUND OF THE INVENTION

[0004] 1. Field of the Invention

[0005] This invention relates to linear and rotary actuators and, inparticular, to non-electromagnetic linear and rotary actuators.

[0006] 2. Description of Related Art

[0007] Linear actuators find widespread applications in industrial,commercial, vehicular, and domestic settings, in uses ranging widelyfrom electric door locks and windshield wipers in automobiles to pinpullers and shutter controllers in mechanical designs. Generallyspeaking, linear actuators comprise solenoid devices in which anelectromagnet is used to translate an armature, and the retraction orextension of the armature is operatively connected in a mechanism toperform useful work. Such devices are commodity items that aremanufactured in many sizes, force/stroke outputs, and AC or DCoperation.

[0008] Despite their widespread adoption, electromagnetic linearactuators have several important drawbacks that require designaccommodations in mechanical systems. Due to the use of electromagnetismas the motive force, these devices necessarily require ferromagneticmaterials to define the armature as well as a magnetic flux circuit tomaximize the stroke force. Such materials are typically dense, and theiruse results in devices that are rather large and heavy, particularly incomparison to their stroke/force output characteristics. Moreover, themultiple turns of wire that comprise an electromagnet, typicallyhundreds or thousands, add another substantial mass to the device.

[0009] Another drawback of electromagnetic linear actuators is also dueto the use of electromagnetism as the driving force. Typically, as thearmature is extended from the electromagnetic, increasing portions ofthe armature are removed from the influence of the electromagneticfield, and the driving force is concomitantly reduced. As a result, theforce versus stroke displacement characteristics of these devicesgenerally exhibit high initial force values that decline rapidly withincrease in stroke displacement. In many mechanisms it is desirable todeliver a constant force linear stroke, and it is necessary to designadditional mechanisms to make use of the negatively slopedforce/displacement characteristic.

[0010] In recent years much interest has been directed toward shapememory alloy (hereinafter, SMA) materials and their potential use inlinear actuators. The most promising material is nickel titanium alloy,known as Nitinol, which, in the form of a wire or bar, delivers a strongcontraction force upon heating above a well-defined transitiontemperature, and which relaxes when cooled. Assuming the Nitinol wire isheated ohmically or by extrinsic means, there is no need for theferromagnetic materials and numerous windings of the prior artelectromagnetic linear actuators, and there is the promise of alightweight linear actuator that delivers a strong actuation force.Moreover, the force versus displacement characteristic of SMA is muchcloser to the ideal constant than comparable electromagnetic devices.

[0011] Despite the great interest in SMA actuators and many forms of SMAactuators known in the prior art, no practical SMA actuator mechanismhas proven to be reliable over a large number of operating cycles. Ithas been found that Nitinol wire requires a restoring force to assistthe material in resuming its quiescent length when its temperature fallsbelow the material's transition temperature. Many prior art SMA actuatordesigns have made use of common spring assemblies, such as helical orleaf springs, to exert the required restoring force. These springassemblies typically deliver a spring force that varies linearly withdisplacement, (F=kx), and the restoring force in most cases is a maximumat maximum stroke. It has been found that the SMA component respondspoorly to this force/displacement characteristic, and the useful life ofthe SMA actuator is severely limited by such a restoring force. Toovercome this problem, prior art designers have attempted to use simpleweights depending from pulleys to exert a constant restoring force onthe SMA component. Although more effective, this expedient results in amechanism that is not easily realized in a small, widely adaptivepackage.

[0012] Another drawback inherent in known SMA materials is therelatively small amount of contraction that is exerted upon heating pastthe transition temperature. The maximum contraction is about 8%, and theuseful contraction for repeated use is about 6%. Thus, to achieve adirect displacement stroke from the SMA component of about one inch, theSMA component must be over sixteen inches long. This material limitationresults in a minimum size that is too large for many applications. Someprior art designs overcome this problem by wrapping the SMA wire aboutone or more pulleys to contain the necessary length within a shorterspace. However, the SMA wire tends to acquire some of the curvature ofthe pulleys as it is repeatedly heated and cooled, and loses too much ofits ability to contract longitudinally. The result is failure after afew number of operating cycles. Other prior art designs employ leverarrangements or the like to amplify the SMA displacement, with aconcomitant reduction in output force.

[0013] It is evident that the prior art has failed to fully exploit thefull potential of shape memory alloy, due to the lack of a mechanismthat capitalizes on the useful material characteristics of SMA.

BRIEF SUMMARY OF THE INVENTION

[0014] The present invention generally comprises a linear actuator thatemploys a shape memory alloy component to deliver a relatively longstroke displacement and reiterative operation over a large number ofcycles.

[0015] In one aspect, the invention provides a plurality of SMAsub-modules, each capable of displacement upon heating of the respectiveSMA component. The sub-modules are linked in a serial mechanicalconnection that combines the stroke displacement of the sub-modules inadditive fashion to achieve a relatively long output stroke. Moreover,the sub-modules may be assembled in a small volume, resulting in anactuator of minimal size and maximum stroke displacement.

[0016] The sub-modules may be fabricated as rods or bars adapted to bedisposed in closely spaced adjacent relationship, each rod or bar linkedin serial mechanical connection to the adjacent rod or bar.Alternatively, the sub-modules may comprise concentric motive elements,with the serial mechanical connection extending from each motive elementto the radially inwardly adjacent motive element, whereby the innermostmotive element receives the sum of the translational excursions of allthe motive elements concentric to the innermost element. For all thesub-module embodiments, the serial links therebetween are provided byone or more shape memory alloy wires, each wire connected at opposedends of adjacent sub-modules to apply contractile force therebetween.

[0017] In another aspect, the invention provides an SMA linear actuatorassembly employing a spring assembly that is designed to apply arestoring force tailored to optimize the longevity of the SMA component.In one embodiment of the spring assembly, a roller/band spring(hereinafter, rolamite) is connected to the output shaft of the linearactuator assembly. The rolamite spring exerts a restoring forcecharacterized by a decrease in force with increasing displacement, sothat the SMA components are returned to their quiescent form with aminimum of residual strain. In a further embodiment, the spring assemblyis comprised of a bar or rod connected to the output shaft of the SMAactuator assembly and confined in a channel for longitudinal translationtherein. The bar includes shaped cam surfaces extending longitudinallytherealong, and a cam follower extends from the channel and isresiliently biased to engage the cam surfaces. As the bar is translatedby actuation of the SMA linear actuator assembly, the cam followerexerts a restoring force that is a function of the slope of the camsurface and the magnitude of the resilient force on the cam follower. Byappropriate shaping of the cam surface, the assembly exerts on the SMAlinear actuator assembly a restoring force characterized by a decreasein force with increasing displacement, whereby the number of cycles ofoperation is maximized.

[0018] In a further aspect, the invention includes a housing in which aplurality of drive rods are arrayed in generally parallel, adjacentrelationship and supported to translate freely in their longitudinaldirections. One end of each drive rod is connected to the opposed end ofan adjacent drive rod by an SMA wire, defining a series of driveassemblies connected in additive, serially linked chain fashion. At oneend of the chain, the drive assembly is joined by an SMA wire to thehousing, and at the other end of the chain, the housing is provided withan opening through which an actuating rod may extend. Also secured inthe housing is a spring, such as a rolamite roller/band spring, havingone end connected to the housing and the other end connected to theactuator rod. The spring is designed to exert a restoring force having aconstant or negative force versus displacement relationship.

[0019] Each SMA wire is connected in an electrical circuit, in one ofseveral arrangements of series or parallel connections, so that ohmicheating may be employed to heat the SMA wires beyond their phasetransition temperature. In the chain-connected series of SMA driveassemblies, the resulting contraction of the SMA wires is cumulative andadditive, and the actuating rod is driven to extend from the housingwith a high force output. When the current in the circuit is terminated,the SMA wires cool below the transition temperature, and the springrestores the SMA wires to their quiescent length by urging the actuatingrod to translate retrograde and (through the chained connection ofassemblies) to apply sufficient tension to re-extend all the SMA wires.

[0020] It may be appreciated that the SMA wires remain in substantiallylinear dispositions throughout the contraction/extension cycle, so thatflex-induced stresses are avoided. To assist in heat removal for highpower applications, the housing may be filled with oil or other thermalabsorber, which may be cooled passively or actively. To deliveradditional force, two or more SMA wires may be connected between thedrive assemblies, rather than one wire. To provide enhanced actuationand retraction times, the SMA wires may be thinner. Although theinvention is described with reference to the shape memory componentcomprising a wire formed of Nitinol, it is intended to encompass anyshape memory material in any form that is consonant with the structureand concept of the invention.

[0021] In addition, each rod or bar is comprised of a link having across-sectional configuration particularly adapted for SMA wire-driven,displacement multiplied applications. In one embodiment, all links havea trigon configuration comprised of an equilateral triangular crosssection defining identical longitudinally extending planar faces. A wiregroove extends longitudinally to bisect each planar face. The triangularvertices are radiused to blunt any sharp edge or wear point. Each planarface is provided with an inset crimp pocket that is dimensioned toreceive and retain a crimp applied to one end of an SMA wire.

[0022] The trigon configuration provides stiffness and bend-resistancegreater than a rectangular beam of similar mass and length. Moreover, itfacilitates a range of stacking patterns that enables a wide range ofoutput stroke or force from a rather small actuator assembly. The linksmay be arrayed in abutting (planar face to planar face) contact andsurrounding a central output shaft, which may have an hexagonal orrhomboidal cross-sectional configuration. The links may be connectedserially by SMA wires to produce a maximum stroke, or may be grouped inparallel action relationship to provide a shorter, more powerful stroke.The number of links may vary from 8, 10, 12, 14, and 18 link stackingpatterns about the central output shaft. Each stacking patternembodiment is provided with a housing that has a longitudinal inneropening that conforms to the outer configuration of the stacked links,whether rectangular, rhomboidal, hexagonal, or the like. The outersurface of the housing may generally conform to the inner opening, ormay be provided with a rectangular, ovoid, cylindrical, or similarconfiguration.

[0023] In a further embodiment all links have a chevron configurationcomprised of generally rectangular cross-section having substantiallygreater width than height, with short flanges extending longitudinallyalong opposite upper vertices and beveled vertices extendinglongitudinally along opposite lower vertices. A SMA wire groove extendslongitudinally along the midline of the upper surface, and a similar SMAwire groove extends longitudinally along the midline of the lowersurface. A crimp pocket may be formed in one end of the upper surfaceand the opposite end of the lower surface.

[0024] In vertically stacked, abutting relationship, the beveledvertices of each link nest in the flanges of the subjacent link to limitsliding movement only to the longitudinal direction. Each SMA wire iscrimped between the upper surface of one link and the lower surface ofthe superjacent link. The wire grooves of the confronting upper andlower surfaces define a channel through which the SMA wire extends, thelateral confines of the channel defining an intrinsic return mechanism(IRM) that causes the SMA wire to return to substantially 100% lengthupon cooling below the shape transition temperature.

[0025] The SMA wires may be linked in serial stacking order to produce amaximum stroke output at the upper or lower link of the vertical stack.Alternatively, the medial link in the stack may be connected serially tothe series of links above and below it, so that the superjacent linksoperate in parallel to the subjacent links to translate the medial linkin a stroke that is half the maximum possible stroke but twice theforce. The device is housed in a rectangular housing that has a smallend section in comparison to the length of the housing.

[0026] In either the trigon or chevron embodiments, the housing maycomprise a generally tubular member having an inner surface configuredto engage in complementary fashion the outer surfaces of the links thatare arrayed therewithin. A pair of end caps are secured in the opposedends of the tubular member, and one or both end caps is provided with anopening through which the output shaft may extend. At least one end capmay include a keyway to accept a key on the output shaft that resistsrotation of the shaft by the device to which it is coupled.

BRIEF DESCRIPTION OF THE DRAWING

[0027]FIG. 1 is a schematic mechanical diagram depicting the fundamentalcomponents of the shape memory alloy actuator of the present invention.

[0028]FIG. 2 is a cross-sectional elevation of one embodiment of theshape memory alloy actuator of the present invention.

[0029]FIG. 3 is a cross-sectional end view of a negative force constantrolling band spring assembly of the shape memory alloy actuator of thepresent invention.

[0030]FIG. 4 is a plan view of one embodiment of the band spring of therolling band spring assembly depicted in FIG. 3.

[0031]FIG. 5 is a partially cutaway side elevation showing a furtherembodiment of the shape memory alloy actuator of the present invention.

[0032]FIG. 6 is a schematic view of a further embodiment of a negativeforce constant spring assembly of the shape memory alloy actuator of thepresent invention.

[0033]FIG. 7 is a graph depicting force versus displacement fordifferent spring assemblies.

[0034]FIG. 8 is a perspective view of a further embodiment of the shapememory alloy actuator of the present invention.

[0035]FIG. 9 is a top view of the embodiment of the actuator inventiondepicted in FIG. 8.

[0036]FIG. 10 is a side elevation of the actuator invention depicted inFIGS. 8 and 9.

[0037]FIG. 11 is a top view of the assembled drive rods of the shapememory alloy actuator depicted in FIGS. 8-10.

[0038]FIG. 12 is an exploded view of the drive rod assembly of the shapememory alloy actuator depicted in FIGS. 8-11.

[0039]FIG. 13 is an exploded view of the drive rod assembly of the shapememory alloy actuator depicted in FIGS. 8-12, with the drive rods in anextended disposition.

[0040]FIG. 14 is a partial perspective view of a drive rod connection toa shape memory alloy wire, in accordance with the present invention.

[0041]FIG. 15 is a perspective view of a further embodiment of a shapememory alloy actuator employing the drive rod connection assembly shownin FIG. 14.

[0042]FIG. 16 is a cross-sectional end view of a further embodiment of ashape memory alloy actuator of the present invention.

[0043]FIG. 17 is a perspective view of one motive element of the shapememory alloy actuator shown in FIG. 16.

[0044]FIG. 18 is a schematic depiction of one series electrical circuitarrangement for heating the SMA wires of the shape memory alloy actuatorof the invention.

[0045]FIG. 19 is a schematic depiction of a series electrical circuitarrangement for heating paired SMA wires of the shape memory alloyactuator of the invention.

[0046]FIG. 20 is a schematic depiction of another series electricalcircuit arrangement for heating paired SMA wires of the shape memoryalloy actuator of the invention.

[0047]FIG. 21 is a schematic depiction of another series electricalcircuit arrangement for heating paired parallel SMA wires of the shapememory alloy actuator of the invention.

[0048]FIG. 22 is a perspective view of another embodiment of a shapememory alloy actuator employing the drive rod connection assembly shownin FIG. 14.

[0049]FIG. 23 is an enlarged cross-sectioned perspective view of atrigon embodiment of the drive link of the invention.

[0050]FIG. 24 is a series of end views of actuator devices in whichtrigon links of FIG. 23 are combined with a hexagonal output shaft invarious stroke and force combinations.

[0051]FIG. 25 is a series of end views of actuator devices in whichtrigon links of FIG. 23 are combined with a rhomboidal output shaft invarious stroke and force combinations.

[0052]FIG. 26 is a series of end views of actuator devices in whichtrigon links of FIG. 23 are combined in groups of links stacked onopposite sides of a central output shaft that is trigon or rectangular.

[0053]FIG. 27 is an end view of an actuator device in which trigon linksof FIG. 23 are combined in groups of links stacked on opposite sides ofa central rhomboidal output shaft.

[0054]FIG. 28 is an exploded perspective view of an actuator deviceconfigured as shown in FIG. 24 with its housing and end caps.

[0055]FIG. 29 is a perspective view of the actuator device of FIG. 28 inassembled condition.

[0056]FIG. 30 is a perspective view of the trigon links of the actuatorextending from the housing of the device of FIG. 28 or 29.

[0057]FIG. 31 is an enlarged cross-sectioned perspective view of achevron embodiment of the drive link of the invention.

[0058]FIGS. 32 and 33 are perspective views of differing connectionschemes of trigon-based actuators having links in an hexagonal stackingpattern.

[0059]FIG. 34 is a perspective view depicting a vertical stack ofchevron links connected in a center output arrangement.

[0060]FIG. 35 is an enlarged perspective view showing a portion of thevertical stack of chevron links of FIG. 24, shown in exploded style withthe SMA wires connected therebetween.

[0061]FIG. 36 is a perspective view of an SMA actuator device having arectangular tubular housing for enclosing the actuator constructionsshown in FIGS. 26, 34 and 35.

DETAILED DESCRIPTION OF THE INVENTION

[0062] The present invention generally comprises a linear actuator thatemploys at least one shape memory alloy component as the drivingelement. The invention provides relatively long stroke displacement withhigh force, and delivers reiterative operation over a large number ofcycles.

[0063] With regard to FIG. 1, one significant aspect of the invention isthe provision of a plurality of stages or sub-modules 31A-31D that formthe linear actuator motor 30. Each sub-module 31 includes alongitudinally extending rod 32, and end brackets 33 and 34 secured tothe lower end and upper end of the rod 32, respectively. The sub-modules31 are arranged to translate reciprocally in the longitudinal direction.Note that the brackets 33 and 34 are generally parallel and extend inopposed lateral directions. A SMA wire 36A extends from the lowerbracket 33A to an anchor point 37, SMA wires 36B extends from the lowerbracket 33B of sub-module 31B to the upper bracket 34A of sub-module31A, and SMA wires 36C and 36D join sub-modules B to C, and C to D, tocomplete a serial chain connection. The SMA wires 36A-36D are fabricatedto undergo a phase transition upon heating to a predeterminedtemperature to contract approximately 4%-8%. The contractile force andexcursion of each SMA wire, represented by arrows A-D, is appliedbetween the sub-modules 31A-31D, each pulling on the next adjacent one,whereby the contractile excursion of each SMA wire 36A-36D is combinedadditively. Thus the sub-module 31D undergoes the greatest translationwhen all SMA wires contract, as labeled in FIG. 1 as total displacement(stroke). Indeed, the effective length of SMA wire in the mechanism issubstantially equal to the sum of the lengths of all the SMA wires36A-36D. This effective length is achieved in a compact mechanism,without resort to pulleys or other bending of the SMA wires.

[0064] The longitudinal rod 32D may be provided with an extended distalend 38 to facilitate delivering the output of the actuator 30 to operatea mechanism or perform other useful work. The SMA wires may be heated byconnecting them in an electrical circuit that directs a current throughall the SMA wires for ohmic heating. The circuit may extend from anegative terminal to bracket 33D, and thence through SMA wire 36D to theadjacent sub-module 31C, and so on to a positive connection at anchorpoint 37. In this series connection all wires 36 are heated at the sametime and, due to the same current passing through all wires 36, to thesame extent.

[0065] The linear actuator described thus far with respect to FIG. 1will exhibit a limited useful life (one or a few cycles of contractionand extension), due to the fact that SMA wire will not relax fully whencooled below the phase transition temperature, unless a restoring forceis applied in the extension direction. To provide a restoring force, aspring 39 is connected at one end to the bracket 34D of sub-module 31D,and the other end is secured to a fixed structural point. The spring 39is arranged to be extended by outward movement of the bracket 34D, thusundergoing extension that increases as the wires 36 contract. When thewires are cooled and contract, the spring restoring force applied to thebracket 34D is applied equally through the linked sub-modules 31 to allthe SMA wires 36. This restoring force aids the SMA wires in returningsubstantially fully to their original length, thus greatly lengtheningthe useful life of the mechanism 30. Preferred embodiments of the spring39 are described in the following specification, although standard formsof coil, leaf, or elastomer springs will suffice for a limited usefullife of the mechanism 30.

[0066] With regard to FIG. 2, the invention may provide a block-likehousing 41 for securing the sub-modules 31 in a compact assembly. Thehousing includes a plurality of passages 42 extending therethrough ingenerally parallel arrangement to permit the longitudinal rods 32A-32Dto extend therethrough. Likewise, a plurality of passages 43 extendparallel and interspersed with the passages 42, to receive the SMA wires36A-36D therethrough. The passages 42 are dimensioned to permit freelytranslating motion without any significant lateral movement, and thepassages 43 are dimensioned to receive the SMA wires with clearance toeliminate contact. The array of passages 42 and 43 is laid out to acceptthe sub-modules 31A-31D in serial linked fashion, as described above,and this layout may be in a linear arrangement or in a curved plane thatcontains all the axes of the passages 42, further foreshortening theouter dimensions of the housing 41.

[0067] With regard to FIG. 5, a further embodiment of the inventioncomprises a linear actuator 51 having an outer shell-like housing 52defined by front, rear, top, and bottom walls 53-56, respectively, in atrapezoidal configuration, and side walls 57 (only one shown in thecutaway view) extending therebetween to form a closed interior space. Aplurality of track elements 58 are supported on both side walls 57 inparallel arrays that define slots extending longitudinally in aparallel, vertically spaced arrangement. A plurality of drive bars 59are provided, each supported in one of the slots defined by the trackelements 58 and received therein in freely translating fashion in theirlongitudinal direction. The drive bars 59 are disposed in a verticallystacked array, and may extend distally or retract proximally along theslots in which they are supported.

[0068] A plurality of SMA wires 61 is provided, each extending betweenand connected to the proximal end of one drive bar 59 and the distal endof the vertically superjacent drive bar. At the top of the verticallystacked array of drive bars, the SMA wire 61 is connected at its distalend to an anchor point 62. At the bottom of the vertically stacked arraythe drive bar 59′ is provided with an elongated distal end that isaligned with a window 63 in end wall 53, through which it may extend.The SMA wires 61 may be heated to a temperature above the phasetransition temperature to contract the wires 61. (Electrical wireconnections are not shown for simplification of the drawing.) Each drivebar 59 is advanced incrementally, as shown by the arrow at the distalend of each bar 59, and, since each wire 61 is anchored in thesuperjacent moving bar, the incremental translation of each bar isapplied to the subjacent bar. Consequently, the lowermost bar 59′undergoes the greatest longitudinal translation, extending through theopening 63 to perform useful work.

[0069] The SMA wires undergo a contraction of approximately 4%-8%. Inthe embodiment of FIG. 5, the configuration of the SMA wires determinesthat the contractile force is exerted substantially along thelongitudinal directions of the drive bars 59, and that the angle of theforce vector does not change appreciably between the contracted andextended states of the wires 61.

[0070] A spring assembly 64 is disposed below the lowermost drive bar59′, and is attached thereto to apply a restoring force to bar 59′ andthus to all the SMA wires 61. The spring assembly 64 comprises arolamite spring, known in the prior art and described fully in SandiaLaboratory Report no. SC-RR-67-656, and available from the Clearinghousefor Federal Scientific and Technical Information of the National Bureauof Standards. Briefly, the spring consists of a pair of rollers 65retained within chamber 66, and a band spring 67 that is passed aboutboth of the rollers 65 in an S configuration. The band spring 67includes a tongue 68 extending therefrom through opening 69 and securedto the drive bar 59′. The rolamite spring tongue exerts a specified,engineered restoring force on the bar 59′ to assure that all the SMAwires 61 return to their fully extended disposition when the wires 61are cooled below their shape memory transition temperature.

[0071] As shown in greater detail in FIGS. 3 and 4, the band spring 67preferably is provided with an internal cutout 71 in an extended Uconfiguration to define the longitudinally extending tongue 68. Thechamber 66 is defined by upper and lower walls 72 and 73, respectively,to constrain vertical movement of the rollers. Side walls 74 (only oneshown) join the upper and lower walls, and constrain lateral movement ofthe rollers 65, so that the rollers 65 may move only longitudinally inthe chamber 66. The band spring 67 is secured at a proximal end to theinner surface of the lower wall 72, and is passed about the two rollers65 in an S configuration, as evident in FIG. 3. The distal end of theband spring 67 is secured to the inner surface of the upper wall 73, andthe tongue 68 diverges from the S configuration to extend through thewindow 69 to join the drive bar 59′. As the tongue 68 extends from theopening 69 it pulls the band spring 67 distally, causing the rollers toroll on their respective portions of the band spring as they translatedistally. The spring return force exerted on the tongue 68 is directlyrelated to the difference between the energy liberated as portions ofthe band unbend versus the energy required to bend other portions of theband when the two rollers translate longitudinally. By selectivelyvarying the width of the band spring 67, or selectively varying thewidth of the cutout 71, it is readily possible to generate a springreturn force that follows a predictable mathematical function.

[0072] As depicted graphically in FIG. 7, a typical prior art helicalspring or leaf spring develops a restoring force F that varies generallylinearly with displacement x, or, F=−kx. For a rolamite spring, thefunction that relates spring return force with displacement may differsignificantly from a typical coil spring or leaf spring. In particular,for restoring the SMA linear actuator mechanisms described herein, ithas been found that the optimal force for restoring the SMA wires tofull extension is one having a negative force constant; i.e., therestoring force decreases as extension of the spring increases. Thisforce characteristic preserves the shape memory effect to the maximumextent, and results in a useful working life (in terms of total numberof cycles of operation) in the same range as typical prior art linearactuators.

[0073] In other words, the slope of the graph representing the springfunction exhibits a negative slope in at least a portion of the springexcursion. If the negative slope is constant, the graph will be linearand parallel to line A of FIG. 7. The negative slope may change atdifferent spring sections, producing a graph B comprised of severalcontiguous linear segments. Or the negative slope may vary continuously,producing a smoothly curved graph of the spring function, as representedby graphs C and D. (The band spring may also be fashioned to definepositive slope areas, discontinuous spring functions, detent and dwellportions, neutral spring force, and the like, as required to providethese desired mechanical functions.)

[0074] It should be noted that the contractile force of the SMA wirephase transition is substantially constant as contraction takes place.As a result, the force delivered by the linear actuators describedherein is substantially constant throughout the outward excursion of theactuator. This desirable characteristic is in marked contrast to typicalsolenoid actuators, which produce maximum force at initial actuation andtaper off significantly as translation progresses.

[0075] With regard to FIG. 6, a further embodiment of a return springhaving a having a negative force constant; i.e., the restoring forcedecreases as extension of the spring increases. A bar or similar movingelement 76 is disposed in a channel 77 and is constrained to translatelongitudinally therein, as shown by arrow L. The element 76 includes aside surface 78 defined by contiguous surface portions 78A-78C thatcomprise a camming surface. A cam follower 79 is comprised of atelescoping mounting for a roller and a spring for urging the roller toengage the camming surfaces 78A-78C. The roller is mounted to roll alongthe camming surfaces as they translate along the channel in thelongitudinal direction. On an opposed side of the element 76, arectangular cutout portion 81 defines a linear, longitudinal surface 82engaged by a cam follower 79′. The cam follower 79′ is provided to applya lateral force to the element 76 to counterbalance the lateral forceimparted by cam follower 79, so that the element 76 will avoid becomingjammed in the channel 77.

[0076] It may be appreciated that the resilient force impinging camfollower 79 into camming surfaces 78 is resolved by classical mechanicstechniques into vector forces exerted longitudinally and laterally onthe element 76. The lateral forces are offset by the follower 79′ andthe channel constraints, so that the longitudinal force component urgesthe element 76 to translate longitudinally, thereby constituting arestoring force. For example, as the element 76 translates distally (tothe right in FIG. 6), the cam follower 79 encounters the steeply angledcam surface portion 78B, and exerts a strong, substantially constantlongitudinal restoring force. When the cam follower 79 progresses andimpinges on the camming surface portion 78A, the restoring force isdecreased to a lower constant due to the shallower slope. (The surface78 may comprise any number of segments, curves, or other features.) Asthe element translates proximally under the urging of the cam follower79, the portion 78C acts as a stop to prevent further proximaltranslation. The spring assembly is capable of generating any desiredrestoring force function.

[0077] With regard to FIGS. 8-13, a further embodiment of the linearactuator of the present invention includes a housing 91 having agenerally rectangular exterior and defining a rectangular interior space94 extending longitudinally therein. A bottom plate 93 and a top plate92 close the opposed ends of the space 94, and the output plunger 95 ofthe actuator extends longitudinally through the central hole 97 of thetop plate. Within the space 94 a matrix of drive rods 96 is disposed inclosely packed array, the dimensions of the space 94 and the closespacing of the rods 96 constraining the rods 96 to be translatable onlyin the longitudinal direction. The rods 96 are formed as rectangularparallelepipeds, with each longitudinally extending rectangular surfaceof each rod being adapted to receive and secure one SMA wire, asdetailed below. This construction enables any two rods 96 in the matrixto be connected together, end to opposite end, whether they arelaterally or vertically adjacent (as viewed in FIG. 11. The matrix alsoincludes a spring housing 98 occupying the space of one drive rod 96, asshown in FIGS. 11 and 12, and enclosing any form of return springdescribed herein. The drive rod 96G at the center of the matrix supportsthe output plunger 95, and is connected to the spring within the housing98, so that the spring applies a restoring force to all the SMA wiressufficient to restore the wires to their original length when cooled.

[0078] Drive rod 96A may be connected at its lower end to an SMA wirethat is connected at its upper end to the housing 91. The upper end ofrod 96A is connected to an SMA wire that extends to the lower end of rod96B. Likewise, rod 96B is connected to rod 96C, and so forth to rods96D-96G, which supports the output plunger 95. When all the SMA wiresare actuated, the drive bars 96A-96G extend in additive fashion, asshown in FIG. 13, to push the plunger 95 longitudinally with a strong,constant force. Although the array of drive bars 96 is depicted as a[3×3] matrix, the arrangement may take the general form of any [M×N]array.

[0079] With regard to FIG. 14, the drive bars 96 include at least one ofthe longitudinally extending channels 101-104, each disposed in one ofthe four longitudinally extending rectangular faces of theparallelepiped configuration. Each channel 101-104 is dimensioned toreceive and secure one SMA wire 106. The wire 106 is provided with amounting die 107 crimped to each end thereof, and a retaining pin 108extends across the end of the channel to pinch the die 107 between thepin 108 and the sloped bottom surface at the end of each channel. Theopposed ends of each pin 108 are secured in a passageway 109 extendingfrom opposed sides of the bar 96 and intersecting the channel 101. Theprovision of the channels 101-104 on each face of the bar 96 enables theconnection of any bar 96 to any adjacent bar 96, whether verticallystacked or laterally adjacent. Each channel 101-104 may be prepared asdescribed with reference to channel 101 to effect interconnection of theadjacent bars 96. The channels 101-104 enable the wires 106 to extendbetween the opposite ends of adjacent impinging bars 96 without anycontact or mechanical interference imparted to the wires by the bars.

[0080] The crimped die 107 is formed of a conductive metal, and theengagement of the pin 109 enables electrical connection to the wires 106by the simple expedient of securing the connecting wires to the outerends of the pins 109.

[0081] With regard to FIG. 15, a further embodiment of the linearactuator of the invention makes use of a drive bar 96 as shown anddescribed with reference to FIG. 14. In this embodiment the bars 96 areprovided with top and bottom channels 102 and 103, and are verticallystacked to be linked in serial, additive fashion as describedpreviously. The vertical stacks (two shown, but any number is possible)are supported by side panels 111 and 112, the side panels supporting atleast one circuit board 113 that controls the application of current tothe SMA wires of the vertical arrays. Conductors 114 extend from eachcircuit board to the mounting pins 108 of the adjacent drive barvertical stack to complete circuits through the SMA wires.Alternatively, the circuit board may provide brush contacts that engagesliding contact pads placed on the drive bars 96. In this embodiment thetopmost drive bar undergoes the additive translation of all thesubjacent bars, as described previously.

[0082] A further embodiment of the return spring 39 is shown in FIG. 22with reference to the embodiment depicted in FIG. 15. However, thisspring construction may be employed with any of the linear actuatorembodiments described herein. Drive bar 96′ at the upper end of thevertically stacked array of drive bars 96 undergoes the maximumlongitudinal displacement, and operates the output plunger (not shown)of the array. A base plate 121 joins the side panels 111 and 112 belowthe array of drive bars. A deflection pin 122 extends laterallyoutwardly from drive bar 96′, and an elastically deformable beam 123extends upwardly from the base plate 121 adjacent to the verticallystacked array, with the upper end of the beam disposed to impinge on thedeflection pin 122 when the actuator is retracted. When the SMA wiresare heated and contract, the longitudinal translation of bar 96′ drivesthe deflection pin 122 to bend the beam 123 elastically, therebyexerting a restoring force on the bar 96′ and on the array of drive barsconnected thereto. The beam 123 may be shaped with a non-uniformcross-section, or provided with other aspects that provide a returnforce function that approximates the spring functions A-D of FIG. 7sufficiently closely to provide full return of the SMA wires to theirelongated state, and also a high number of repetition cycles.

[0083] With reference to FIGS. 16 and 17, a further embodiment of thelinear actuator of the invention includes a plurality of drive modules126, each comprising a tubular member of rectangular cross-section,although circular and polygonal cross-sections are equally usable. Thedrive modules 126 are dimensioned to be disposed in concentric, nestedfashion with sufficient clearance for telescoping translationtherebetween. Each drive module 126 includes a plurality oflongitudinally extending projections 127, each projection 127 extendingfrom a medial end portion of one side of the respective drive module126, as shown in FIG. 17. (For a cylindrical tubular array, theprojections are spaced at equal angles about the periphery of the end ofeach drive module.)

[0084] Each side of each drive module 126 is provided withlongitudinally extending channels 101 and 103 on the outer and innersurfaces, respectively, the channels being constructed as described withreference to FIG. 14. Each projection 127 supports a mounting pin 108received in aligned holes 109 to retain the crimped die 107 of an SMAwire 106, as described previously. The inner channel 103 providesclearance for the SMA wire of the nested drive module disposedconcentrically within. The number of SMA wires used may vary; in theembodiment shown in FIG. 17, at least two SMA wires 106 are used atradially opposed sides of the nested modules to provide balancedcontractile forces that resist binding of the telescoping elements. FourSMA wires per module may be used, one secured to each projection 127, toprovide maximum contractile force and maximum force to the actuatingplunger. A return spring assembly, of any construction discussed herein,may be placed within the inner cavity of the innermost concentricelement 126 and connected between the innermost and outermost elements126.

[0085] The SMA wires 106 of any contractile array described herein maybe connected for ohmic heating by any of the circuit arrangementsdepicted in FIGS. 18-21. In these Figures, each drive element 140 mayrepresent any of the drive bars or drive modules 32, 59, 96, or 126described previously. Single SMA wires 141 are connected at like ends ofthe elements 140 by extendable wires (or sliding brush contacts) 146 toform a continuous series circuit that includes all of the SMA wires 141.The moving end of the series circuit is connected to lead wire 143 andthe other end, which is fixed in anchor point 142, is connected to lead144 of the current source that actuates the array. This circuitarrangement assures that all wires carry the same current.

[0086] With regard to FIG. 19, a pair of SMA wires 141 are extendedbetween each pair of drive elements 140, thereby multiplying the forceoutput. This arrangement is depicted in the embodiments of FIGS. 15-17and 22, although all embodiments may support multi-wire arrangements.The paired SMA wires are electrically isolated each from the other, andextendable wires 147 (or sliding brush contacts) are secured to the likeends of the paired SMA wires, so that each pair of SMA wires isconnected in series. The series pairs are likewise connected in a serieselectrical circuit by extendable wires 148, with lead wires 143 and 144extending from the same end of the array. (It may be appreciated thatthe number of wires extending between adjacent drive stages may be anyinteger number other than two.) This arrangement provides multipliedforce output using a series circuit to actuate the wires.

[0087] With regard to FIG. 20, a further embodiment of the electricalactuating circuit of the invention includes paired SMA wires 141extending between adjacent drive elements 140, the paired wires beingelectrically isolated each from the other. Extendable wires 151interconnect the SMA wires so that each SMA wire of each pair isconnected in series with one of the SMA wires of the adjacent driveelement. Thus the circuit is comprised of two series branches thatextend from the anchor point 142 to the proximal end of the output driveelement 140, where they are bridged by connection 152. This connectionarrangement provides multiplied output force and, most notably, bothleads 143 and 144 from the power circuit are connected at the fixedanchor point 142, so that the leads are not connected to a movingobject.

[0088] Another embodiment of the actuating circuit, depicted in FIG. 21,also makes use of paired SMA wires 141 extending between adjacent driveelements 140. In this arrangement each pair of SMA wires is connected inparallel, and the paralleled wires are connected by extendable wires 154in a series circuit. Lead 144 connects to the anchor point of the array,and lead 143 is connected at the proximal end of the output driveelement 140. This circuit arrangement provides the multiplied forceoutput from a current draw that is double that of the previousembodiments.

[0089] Previous embodiments, such as those shown in FIGS. 15 and 22,depict electrical power connections from the circuit board to each drivebar assembly. This feature permits any of the connection schemesdescribed above, and also permits direct connection to each SMA wire forindividual actuation thereof. Thus actuation of the SMA wires may becarried out simultaneously, or staged sequentially in individual orgrouped actuations.

[0090] With regard to FIG. 23, a further embodiment of the drive rodcomprises a trigon link 201. Each trigon link 201 is provided with anequilateral triangular cross-section defining identical longitudinallyextending planar faces 202. A wire groove 203 extends longitudinallyalong the bisector line of each planar face. The vertices formed by theintersections of the planar faces are radiused or otherwise blunted 204to prevent undue wear. Each planar face 202 is provided with a recessedcrimp pocket 206 at one end of each wire groove 203. A crimp 207 issqueezed onto one end of an SMA wire 208 and driven into the crimppocket 206 to secure the wire end to the link. The other end of the wire208 is led to the confronting planar face of an adjacent link 201 whereit is joined by a crimp driven into a similar crimp pocket. This linkageis reiterated among adjacent links to form a displacement multiplied SMAmechanism, in which the displacement of each SMA wire is multiplied bythe number of wires connected between the links, as described above inthe previous embodiments. (It is noted that other techniques forattachment of the SMA wire to the link may be used, such as laserwelding, ultrasonic welding, spot welding, and the like.)

[0091] The grooves 203 are dimensioned so that confronting grooves ofadjacent links combine to form a channel that guides a SMA wire 208extending therein. This arrangement provides an intrinsic returnmechanism to restore the links to their quiescent (retracted) positionwith little or no added spring force required. This IRM is describedelsewhere in this disclosure, and is covered in detail in copending U.S.patent application Ser. No. 10/200,672, filed Jul. 7, 2002. It isconsidered a companion application and is incorporated herein byreference.

[0092] The link 201 may be fabricated of a conductive metal such asaluminum, which is then anodized to form an electrically insulatingoxide coating on all surfaces. Alternatively, the link may be fabricatedof a conductive metal that is then coated or overmolded with aninsulating compound that may also define some or all of the surfacefeatures described above. The crimp pocket 206 may be milled or punched,and the crimp-to-pocket contact is electrically conductive. Thus anelectrical path is formed through each link 201 and the crimp and SMAwire secured thereto, to the adjacent link to which the SMA wire isconnected, and so on.

[0093] One benefit of the trigon configuration is the variety ofstacking patterns that it makes possible. With regard to FIG. 24, acentral output shaft 211 having a hexagonal cross-section is provided,each facet of the hexagon being configured similarly to the planar faces202. (In FIG. 24, each SMA wire 208 is viewed on end as a black dot.Each black dot 1) adjacent to a hexagonal facet indicates where a linkis joined to the shaft; 2) at an outer face of a link indicates where alink is joined to the housing; 3) between two confronting faces 202indicates the serial joining of the two respective links.) Applying alink 201 face-to-face to each hexagonal facet, and filling in thetriangular voids therebetween with other links 201 results in 18 linkssurrounding the output shaft 211. When all the links are joined inserial fashion by SMA wires 208, and the eighteenth link is joined tothe housing (described below) by a SMA wire 208, the result is thedisplacement multiplication of 19 SMA wires (labeled 1×19 in FIG. 24),yielding a substantial output stroke for the shaft 211. In theconfiguration labeled 2×10, there are two groups of nine links that arejoined serially, with one end link of each group joined to the housingand the other end link of each group joined to the output shaft 211. Theresult is a stroke that is approximately half as long and twice asforceful as the 1×19 configuration. Likewise, the links 201 may beconnected in 3×7 or 6×4 or 4×6 configurations to produce strokes thatare shorter but even more forceful. These different configurations,which have the same hexagonal outer conformation, provide a wide rangeof output stroke and force that enables the actuator to be adapted tovarious uses and mechanisms. Of course, the connection of an SMA wire tothe housing may be eliminated in any of these configurations tofacilitate assembly.

[0094] With regard to FIG. 31, a trigon link array having a 4×6 schemeof serial and parallel connections in which groups of 3 adjacent linksare joined from the housing at one end of the serial chain to the outputshaft 211 at the other end of the chain. This arrangement involves 6serial chains, each connected between the housing and the shaft 211 andcapable of extending it a distance equal to four SMA wire incrementalcontractions. Note that trigon links A1, A2, A3 are laterally adjacentto each other and that A2 extends incrementally beyond A3, and that A1extends incrementally beyond A2, and that the shaft 211 extendsincrementally beyond link A1. This is a clear indication that A1-A3 arelinked in series from the housing to the shaft 211 and generating adisplacement equal to 4 SMA wire contractions. Likewise trigon linksB1-B3 are arrayed in descending order from the shaft, and so on withlinks C1-C3, and D-F series connections (not shown).

[0095] With regard to FIG. 32, the same output shaft 211 and an equalnumber of trigon links may be connected in a 9×2 configuration in whichtwo groups of 9 links each are joined mechanically and electrically, andeach group is connected between the housing and the output shaft. Herethe adjacent links A1-A9 are connected in descending serial order fromthe shaft 211 to the housing, as are links B1, B2, etc. The resultingstroke of shaft 211 is equal to nine SMA wire contractions, more thantwice the previous example of FIG. 31.

[0096] With regard to FIG. 25, a central output shaft 212 having arhomboid cross-section is provided, each facet of the rhomboid beingconfigured similarly to the planar faces 202. This arrangement enablesthe stacking of fourteen links 201 about the shaft 212, with each facetof the rhombus in face-to-face contact with a link 201 and the outerconfiguration of the link stack being rhomboidal. When all the links arejoined in serial fashion by SMA wires 208, and the fourteenth link isjoined to the housing (described below) by a SMA wire 208, the result isthe displacement multiplication of 15 SMA wires (labeled 1×15 in FIG.25), yielding a substantial output stroke for the shaft 212. Anotherpossible connection pattern is 2×8, yielding a stroke that isapproximately half as long and twice as forceful. It is also possible toprovide some links 201 that are not connected in the displacementmultiplied scheme, but are placed merely to fill space in the stackingpattern. As shown in the 4×4 embodiment of FIG. 25, two links 213 arenot connected mechanically or electrically, and the remaining links arejoined in four groups of three, with the outermost wire of each groupconnected to the housing, so that the displacement sum is four and thestroke force is four times that of a single SMA wire. Alternatively, asshown in FIG. 27, a rhomboidal output shaft 212 may be surrounded by tenlinks 201, connected in two groups of five and capable of a 2×6 outputstroke.

[0097] The links 201 may also be stacked in other arrangements. As shownin FIG. 26, an output shaft 215 having a rectangular cross-section maybe disposed intermediate of two groups of four links 201, resulting in a2×5 drive configuration. Likewise, an output shaft 214 that is itself atrigon link may be disposed intermediate of two groups of four links201, which also results in a 2×5 drive configuration.

[0098] In all of the connection schemes described above, the fact thateach group of links 201 connected in series has one “outer” SMA wireconnected to the housing or equivalent external structure, and the other“inner” SMA wire at the other end of the group is connected to theoutput shaft (211, 212, 214, or 215). This arrangement enables the“outer” wire to be electrically connected at the fixed point where itjoins the housing (or similar anchoring structure). Given the fact thatthe output shaft may be made conductive, the “inner” wires of the groupsmay be electrically connected at the shaft, so that an electrical paththrough the SMA wires is a circuit connected by fixed (non-moving)electrical connections at opposed ends of the circuit, with no movingcontacts within the circuit. This fact enables reliable connections withno mechanical wear, and the provision of convenient connector terminalsat the exterior of the device housing.

[0099] With regard to FIG. 28, any of the link group arrangements ofFIG. 24 may be assembled into a housing 221 comprised of a tubular case222 having a generally cylindrical outer surface 223 and an inner bore224 having a hexagonal configuration. The inner bore is dimensioned toreceive the hexagonal profile of the trigon links 201 assembled to thecentral output shaft 211. An output rod 225 extends axially from theshaft 211. A pair of end caps 226 are provided with inner ends 227adapted to be received in the bore 224, and a central passage 228through which the rod 225 may translate reciprocally as the device isactuated and deactuated. Set screws 229 may be used to secure the endcaps in tapped holes in the case 222.

[0100] With regard to FIGS. 29 and 30, the assembled housing and endcaps produces a substantial stroke/force output from a relativelycompact package. Note that the end caps may be identical, or one mayhave passage 228 for the output rod, and the other may not, depending onwhether the output rod is double-ended. In addition, the rod 225 may beprovided with a key 231, and the end cap with a keyway 232, so that therod cannot be twisted to impart torque to the links 201. This keywayrotational lock may be applied to any of the actuator devices describedherein. Alternatively, the rod 225 may be provided with anon-cylindrical geometric shape (triangle, hexagon, octagon, etc.), andthe passage 228 is formed in complementary fashion, so that the shaftcannot be rotated in the passage 228.

[0101] Note that the trigon configuration provides stiffness and bendmoment greater than a rectangular beam of similar mass and length. Atthe same time, it has sufficient depth through the cross-section toenable the placement of a crimp pocket that is deep enough to receive acrimp that does not project beyond the surrounding planar face 202. Thuselectrical and mechanical connections are made that do not interferewith the smooth sliding translation of the links abutting each other andthe housing bore 224.

[0102] With regard to FIG. 33, a further embodiment of the drive rodcomprises a chevron link 251. Each chevron link 251 is provided with agenerally rectangular cross-sectional configuration, with upper andlower longitudinally extending surfaces 252 and 253 extending betweenlongitudinal side walls 254. A pair of longitudinally extending flanges256 extend upwardly along opposite edges of the upper surface 252. Theopposed edges of the lower surface are provided with longitudinallyextending bevels 257. In vertically stacked, abutting relationship, asshown in FIG. 31, the bevels 257 of each link 251 nest in the flanges256 of the subjacent link to limit sliding movement only to thelongitudinal direction.

[0103] Each chevron link is also provided with a pair of wire grooves262 and 263 extending longitudinally along the bisector line of eachplanar surface 252 and 253, respectively. The grooves are dimensioned sothat confronting top and bottom grooves 262 and 263 of verticallyadjacent links 251 combine to form a channel that guides a SMA wire 264extending therein. This arrangement provides an intrinsic returnmechanism to restore the links to their quiescent (retracted) positionwith little or no added spring force required. This IRM is describedelsewhere in this disclosure, and is covered in detail in copending U.S.patent application Ser. No. 10/200,672, filed Jul. 7, 2002. It isconsidered a companion case and is incorporated herein by reference.

[0104] Each link 251 is also provided with a recessed crimp pocket atopposite ends of wire grooves 263 and 263. A crimp is squeezed onto oneend of an SMA wire 264 and driven into the crimp pocket to secure thewire end to the link. The other end of the wire 264 is led to theconfronting planar face of an adjacent link 251 where it is joined by acrimp driven into a similar crimp pocket. (Refer to FIG. 23 for acomparable showing.) This linkage is reiterated among adjacent links toform a displacement multiplied SMA mechanism, as described above in theprevious embodiments. Each link may be conductive and coated with anon-conductive material (such as anodized aluminum, or an overmoldedpolymer or plastic), so that a conductive circuit is established througheach SMA wire, thence through the link to which it is crimped or welded,and thence to the next SMA wire.

[0105] The chevron link 251 is designed to be stacked vertically withthe SMA wires connected between one end of the upper surface of one linkand the other end of the lower surface of the serially superjacent link.As shown in FIGS. 34-36, one possible vertical stacking arrangement oflinks 251 comprises a central output link 261 that is disposed mediallyin the stack of links. The links 251A below the output link 261 areconnected contiguously by SMA wires 262 as shown in FIG. 35, so that thebottom-most link moves one incremental distance (one SMA wirecontraction increment, shown by small arrow) due to that link having anSMA wire connected to the housing at its other end. Each superjacentlink is moved a distance equal to the sum of the incremental movement ofthe subjacent links in the stack plus the increment of its associatedSMA wire (shown by number of small arrows). The four links shown thusdeliver to the output link 261 a stroke equal to five incrementalmovements. Likewise, the four links 251B above the central output link261 are connected contiguously in mirror image to links 251A to drivethe link 261 in the same direction for the same distance with the sameforce, so that during operation of the actuator the output link 261extends outwardly from the middle of the stack as shown in FIG. 34.

[0106] With regard to FIG. 36, a further embodiment of the housing ofthe invention comprises a rectangular tubular member 271 having arectangular interior bore dimensioned to receive a stack of links 251 asshown in FIG. 34, with an output rod 272 extending from the medial link261. The interior bore includes sufficient space for the stack of linksto under displacement multiplied translation, as shown in FIG. 34, andto return to a quiescent position in which the stack is aligned withrespective ends in common planes. A pair of end caps 273 are secured inthe opposed ends of the interior bore, and end cap 273′ is provided witha central passage through which the output rod 272 extends freely. Setscrews 274 may extend through a sidewall of the tubular member 271 tosecure the end caps.

[0107] As mentioned above, a contiguous, serial connection scheme asshown in FIGS. 34 and 34 defines an electrical circuit through the SMAwires 264 and conductive links 251, and the circuit's only two externalcontact points may be fixed at the exterior of the member 271. Forexample, a contact connector 276 (plug or receptacle) may extend fromone side wall of the member 271. It is provided with four connectorposts, which is sufficient for providing power and sensor links (fortemperature, travel limit, and the like). The power posts may be wireddirectly to the ends of the SMA wires at their anchorage in the housing.A similar external connector may be provided in any of the embodimentsdescribed previously with regard to FIGS. 28-30. Note also that thehousing construction of FIG. 36 may be well-suited for the connectionschemes of FIG. 26 using trigon links, or in general links of anygeometric configuration that enables a stacked relationship as describedherein.

[0108] Note that in all the embodiments described herein the channels orgrooves in which the SMA wires are positioned may be dimensioned toconfine the SMA wires to longitudinal movement only, whereby anintrinsic return mechanism is established for the contiguous assembly oflinks. The IRM may obviate the need for a return spring, dependingprimarily on the device to which the actuator is connected and itsresistance to movement in the intrinsic return direction.

[0109] Although the invention is described with reference to the shapememory component comprising a wire formed of Nitinol, it is intended toencompass any shape memory material in any form that is consonant withthe structural and functional concepts of the invention.

[0110] The foregoing description of the preferred embodiments of theinvention has been presented for purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed, and many modifications andvariations are possible in light of the above teaching without deviatingfrom the spirit and the scope of the invention. The embodimentsdescribed are selected to best explain the principles of the inventionand its practical application to thereby enable others skilled in theart to best utilize the invention in various embodiments and withvarious modifications as suited to the particular purpose contemplated.It is intended that the scope of the invention be defined by the claimsappended hereto.

1. A linear actuator, including: a plurality of links disposed inclosely spaced array and adapted to undergo reciprocal translation in alongitudinal direction: an output shaft connected to said plurality oflinks and adapted to undergo reciprocal translation in said longitudinaldirection; said plurality of links each having a common geometric formadapted for stacking in a predetermined array with respect to saidoutput shaft; a plurality of shape memory wires connected to saidplurality of links in serial, contiguous fashion; means for heating saidshape memory wires beyond the memory transition temperature to contractsaid wires and urge said links to move in said longitudinal direction indisplacement multiplied fashion.
 2. The linear actuator of claim 1,wherein said means for heating includes an electrical circuit extendingthrough all of said SMA wires and all of said links.
 3. The linearactuator of claim 2, wherein said plurality of links are divided into aplurality of separate groups, each of said groups being mechanically andelectrically connected by said SMA wires in serial, contiguous,displacement multiplied fashion.
 4. The linear actuator of claim 3,wherein said plurality of separate groups are mechanically connectedbetween a mechanical ground and said output shaft.
 5. The linearactuator of claim 4, wherein said mechanical ground comprises a housingformed to extend about said plurality of links and plurality of wires.6. The linear actuator of claim 4, wherein each of said groups includesa first SMA wire connected from said mechanical ground to a first linkin the group, and a last SMA wire connected from a last link in thegroup to said output shaft.
 7. The linear actuator of claim 6, whereinsaid housing includes at least one electrical connector disposed toconnect to said first SMA wire at said mechanical ground.
 8. The linearactuator of claim 6, wherein all of said groups are electricallyconnected at said output shaft as a common node.
 9. The linear actuatorof claim 3, wherein two separate groups of links are disposed atopposite sides of said output shaft, said output shaft extending andretracting from a medial position in said closely spaced array.
 10. Thelinear actuator of claim 1, wherein said plurality of links each have atrigon configuration comprised of three generally identical planar facesextending in said longitudinal direction and oriented by an equilateraltriangular cross-section of each link.
 11. The linear actuator of claim10, further including a wire groove extending longitudinally along alongitudinal bisector line of each of said planar faces.
 12. The linearactuator of claim 11, further including a crimp pocket formed in each ofsaid planar faces at one end of said wire groove.
 13. The linearactuator of claim 12, further including a crimp applied to one end ofone of said SMA wires and driven into said crimp pocket, said crimpbeing below the surface of the respective planar face.
 14. The linearactuator of claim 13, wherein said crimp forms an electrical connectionthrough said crimp pocket to said link.
 15. The linear actuator of claim14, wherein said links are formed of a conductive material, and saidlinks are coated with an electrically insulating material.
 16. Thelinear actuator of claim 11, wherein said links are disposed inface-to-face abutment in said closely spaced array,
 17. The linearactuator of claim 16, wherein said wire grooves of two of said planarfaces in abutment together define a wire channel for retaining one ofsaid SMA wires in an intrinsic return fashion.
 18. The linear actuatorof claim 10, wherein said output shaft includes a plurality of saidplanar faces extending longitudinally thereon and adapted to abut saidplanar faces of said links in complementary fashion.
 19. The linearactuator of claim 18, wherein said output shaft includes a hexagonalportion wherein each facet of said hexagonal portion comprises one ofsaid planar faces.
 20. The linear actuator of claim 19, wherein saidpredetermined array includes eighteen of said links stacked about saidhexagonal portion and defining an hexagonal outer configuration of saidpredetermined array.
 21. The linear actuator of claim 20, wherein saidplurality of links are mechanically and electrically connected by saidSMA wires in serial, contiguous, displacement multiplied fashion. 22.The linear actuator of claim 21, wherein said plurality of links aredivided into a plurality of separate groups, each of said groups beingmechanically and electrically connected by said SMA wires in serial,contiguous, displacement multiplied fashion.
 23. The linear actuator ofclaim 22, wherein each of said groups includes a first SMA wireconnected from a mechanical ground to a first link in the group, and alast SMA wire connected from a last link in the group to said outputshaft.
 24. The linear actuator of claim 23, wherein said mechanicalground includes a housing formed to extend about said plurality of linksand plurality of wires and said shaft.
 25. The linear actuator of claim24, wherein said housing includes a tubular member having an inner boreconfigured to receive said hexagonal outer configuration of saidpredetermined array of links.
 26. The linear actuator of claim 25,wherein said housing includes at least one electrical connector disposedto connect to said first SMA wire at said mechanical ground.
 27. Thelinear actuator of claim 26, wherein all of said groups are electricallyconnected at said output shaft as a common node.
 28. The linear actuatorof claim 25, wherein said housing includes a pair of end caps secured inopposed ends of said inner bore.
 29. The linear actuator of claim 28,further including a central passage in at least one of said end capsthrough which a portion of said output shaft may extend.
 30. The linearactuator of claim 18 wherein said output shaft includes a rhomboidportion wherein each facet of said rhomboid portion comprises one ofsaid planar faces.
 31. The linear actuator of claim 30, wherein saidpredetermined array includes fourteen of said links stacked about saidrhomboid portion and defining a rhomboidal outer configuration of saidpredetermined array.
 32. The linear actuator of claim 31, wherein saidplurality of links are mechanically and electrically connected by saidSMA wires in serial, contiguous, displacement multiplied fashion. 33.The linear actuator of claim 32, wherein said plurality of links aredivided into a plurality of separate groups, each of said groups beingmechanically and electrically connected by said SMA wires in serial,contiguous, displacement multiplied fashion.
 34. The linear actuator ofclaim 33, wherein each of said groups includes a first SMA wireconnected from a mechanical ground to a first link in the group, and alast SMA wire connected from a last link in the group to said outputshaft.
 35. The linear actuator of claim 34, wherein said mechanicalground comprises a housing formed to extend about said plurality oflinks and plurality of wires and said shaft.
 36. The linear actuator ofclaim 35, wherein said housing includes a tubular member having an innerbore configured to receive said rhomboidal outer configuration of saidpredetermined array of links.
 37. The linear actuator of claim 1,wherein said plurality of links each have a chevron configurationcomprised of a longitudinally extending member having a generallyrectangular cross-section.
 38. The linear actuator of claim 37, whereineach link includes a pair of longitudinally extending flanges projectingoutwardly from laterally opposed edges of an upper surface of each link.39. The linear actuator of claim 38, wherein each link includes a pairof longitudinally extending bevels along laterally opposed edges of alower surface of each link.
 40. The linear actuator of claim 39, whereinsaid links are adapted for vertical stacking in which said pair ofbevels of each link is received in nesting relationship to said pair offlanges extending from the subjacent link in said vertical stack. 41.The linear actuator of claim 40, further including a wire grooveextending longitudinally along a longitudinal bisector line of each ofsaid upper and lower surfaces.
 42. The linear actuator of claim 41,further including a crimp pocket formed in each of said upper and lowersurfaces at one end of said wire groove.
 43. The linear actuator ofclaim 42, wherein said links are formed of a conductive material, andsaid links are coated with an electrically insulating material.
 44. Thelinear actuator of claim 43, wherein said links and said SMA wires areconnected mechanically and electrically in contiguous, serial fashionfor displacement multiplied reciprocal motion.
 45. The linear actuatorof claim 44, wherein said links are formed of a conductive material, andsaid links are coated with an electrically insulating material.
 46. Thelinear actuator of claim 16, wherein said wire grooves of two of saidupper and lower surfaces in abutment in said vertical stack togetherdefine a wire channel for retaining one of said SMA wires in anintrinsic return fashion.
 47. The linear actuator of claim 44, whereinsaid plurality of links are divided into two separate groups, each ofsaid groups being mechanically and electrically connected by said SMAwires in serial, contiguous, displacement multiplied fashion.
 48. Thelinear actuator of claim 47, wherein each of said groups includes afirst SMA wire connected from a mechanical ground to a first link in thegroup, and a last SMA wire connected from a last link in the group tosaid output shaft.
 49. The linear actuator of claim 48, wherein saidmechanical ground includes a housing formed to extend about saidplurality of links and plurality of wires and said shaft.
 50. The linearactuator of claim 49, wherein said housing includes a tubular memberhaving an inner bore configured to receive rectangular outerconfiguration of said predetermined array of links.
 51. The linearactuator of claim 50, wherein said housing includes at least oneelectrical connector disposed to connect to said first SMA wire at saidmechanical ground.
 52. The linear actuator of claim 51, wherein both ofsaid groups are electrically connected at said output shaft as a commonnode.
 53. The linear actuator of claim 50, wherein said housing includesa pair of end caps secured in opposed ends of said inner bore.
 54. Thelinear actuator of claim 53, further including a central passage in atleast one of said end caps through which a portion of said output shaftmay extend.